1. Field of the Invention
The invention relates generally to a lubricant for lubricating journal bearings in a rock bit for drilling earth formations.
2. Background Art
Rock bits are employed for drilling wells in subterranean formations. Such bits have a body connected to a drill string and a single roller cone or a plurality (typically three) of roller cones mounted on the body for drilling rock formations. The roller cones are mounted on journals or pins integral with the bit body at its lower end. In use, the drill string and bit body are rotated in the bore hole, and each cone rotates on its respective journal as the cone contacts the bottom of the bore hole being drilled.
When such a drill bit is used in hard, tough formations, high pressures and temperatures are encountered. The total useful life of a drill bit in such severe environments is in the order of 20 to 200 hours for bits in sizes of about 6 to 28 inch diameter at depths of about 5,000 to 20,000 feet. Useful lifetimes of about 65 to 150 hours are typical. When a drill bit wears out or fails as a bore hole is being drilled, it is necessary to withdraw the drill string to replace the bit, a very expensive process. Prolonging the lives of drill bits minimizes the lost time in “round tripping” the drill string for replacing bits.
Replacement of a drill bit can be required for a number of reasons, including wearing out or breakage of the structure contacting the rock formation. One reason for replacing the rock bits includes failure or severe wear of the journal bearings on which the roller cones are mounted. The journal bearings are lubricated with grease adapted to survive in these severe conditions. Lubrication failure can sometimes be attributed to misfit of bearings or seal failure, as well as problems with the grease.
The journal bearings are subjected to very high drilling loads, high hydrostatic pressures in the hole being drilled, and high temperatures due to drilling, as well as elevated temperatures in the formation being drilled. The journal bearings are often subjected to extremely high loads as a result of the speed of the bit and the weight of the drill string. The operating temperature of the grease in the drill bit can exceed 300° F. Considerable work has been conducted over the years to produce bearing structures and employ materials that minimize wear and failure of such bearings.
A variety of grease compositions have been previously employed. Such grease compositions comprise a generally low viscosity, refined petroleum or hydrocarbon oil basestock which provides the base lubricity of the composition and may constitute about three quarters of the total grease composition. Such basestock oil is thickened with a conventional metal soap or metal complex soap wherein the metal is aluminum, barium, calcium, lithium, sodium, or strontium. U.S. Pat. No. 4,358,384 discloses such a grease composition comprising a petroleum derived mineral oil lubricant basestock and a metal soap or metal complex soap including aluminum, barium, calcium, lithium, sodium or strontium metals. A lighter, lower-viscosity basestock is generally employed to obtain low temperature greases, and a heavier, higher-viscosity basestock is used to obtain high temperature greases.
In order to enhance the film lubricating capacity of such petroleum basestock greases, solid additives such as molybdenum disulfide, copper, lead or graphite have been previously added. Synthetic polymer extreme pressure (EP) agents and high viscosity synthetic polymers may also be used. Such materials serve to enhance the ability of the lubricant basestock to form a friction-reducing film between the moving metal surfaces under conditions of extreme pressure and to increase the load carrying capacity of the lubricants. The function of the lubricant is to minimize wear and to prevent scuffing and welding between contacting surfaces. U.S. Pat. Nos. 4,358,384, 3,062,741, 3,107,878, 3,281,355, and 3,384,582 disclose the use of molybdenum disulfide, and other solid additives such as copper, lead and graphite, which have been employed to attempt to enhance the lubrication properties of oils and greases.
However, the use of solid EP agents, which improve the load carrying capacity, has been shown to contribute to excessive seal and hub wear and drill bit seal failure. For example, drill bit lubricant compounds comprising a copper EP agent have displayed seal failure due to copper deposits and loading near the seal area. The copper accumulates near the seal area until the seal is abraded by the constant and progressive erosive contact with the copper deposit. The abraded seal eventually loses its capacity to retain the grease composition in the journal area, permitting metal to metal contact between the roller cone and the journal, causing drill bit failure. Conversely, lubricants that reduce seal and hub wear typically lack sufficient film strength, that is, load carrying capacity, to be used as a drill bit lubricant.
Additionally, the use of solid EP agents comprising heavy metal complexes are not desirable due to their general toxicity and environmental impact
Accordingly, there exists a need for a lubricant that exhibits both a good load carrying capacity and reduced seal and hub wear.